After bad luck with my first betta (who died of some sort of infection it had when I bought it before I could diagnose it) I decided to take another stab at trying to keep a healthy betta. I just got my betta fish less than a week ago and the first day, she was swimming around and exploring as a normal fish would. The next day she wound down a bit and at a normal amount of food. The next day she was very lethargic, mostly staying in one corner of the tank all day by the heater. I checked to make sure my water was at a good temperature and it was and still sits at about 82 degrees. Also on the third day she didn't eat. I attributed this to the fact that she was stressed out and in a new tank so I thought that I'd give her some time to acclimate. Although, later that night I noticed that her poop was very long and stringy and white and I knew something wasn't normal. The next day, after another failed attempt at eating, I noticed that her belly seemed to be bloated out and she would sometimes curve her body in the shape of a C. I then started looking up what it could possibly be and I figured she was either constipated or had a bladder infection. Ive been doing daily water changed making sure her water is extra clean because I also noticed she had some fin rot from being in her cup in Petco. I started treating the water with Methylene Blue to see if that would do the trick as for something that I haven't been able to diagnose. I've been searching every day to try and figure out what this could possibly be and I just read that white poop is a sign of an internal parasite. I looked through all the emergency medications I had previously bought for my other fish and there doesn't seem to be anything that will treat the internal parasite or bacterial infection directly. Basically after this extremely long story about my freaking out trying to figure out what's wrong with my betta, I just want to know.. How do I treat this? I read somewhere that for internal parasites that Malachite Green works very well, but I couldn't find anywhere in the area that carries that medication. It may not even be an internal parasite, it may be Ich or something and I just can't tell because my fish is a cream color with burgundy fins. (Also in the pictures you can see the white poop again! )

Here is all the technical stuff:
I have a 5G tank with a filter.
I just tested the water and here are the results:
the pH is at 7.2
Alkalinity is 80
Hardness is 75
Nitrite is almost at.5
and Nitrate is at 20

Also like I said I've been doing water changes of about a gallon every day in succession with the Methylene Blue treatments (I've done for two days and tonight will be the final treatment)

Please if anyone could just help me out with how I should go about treating my betta. I've been trying to do my research and keep a good habitat for my betta (and even though this is my second betta fish I've ever gotten) I've had no luck in treating them when they are sick

Also! I'm sorry in anything is inadequate, I just found this site and after reading a few posts, everyone on the site seems to be very helpful. I'm giving this my best go so just let me know if you need to know anything else and I can try to figure it out for you.